The White House has rejected a Turkish offer to release an American pastor in exchange for forgiveness of billions of dollars in US fines on a Turkish state-run bank, expressing that other matters would be considered only after Andrew Brunson is released, a senior administration official told The Wall Street Journal.

Turkish President Recep Tayyip Erdoğan, seeking a gesture in exchange, asked the U.S. to drop an investigation into Halkbank, which is facing potentially crippling fines for allegedly violating U.S. sanctions on Iran, the Journal said.

The White House official told the Journal the U.S. made clear to Turkey that areas of dispute between the two nations, including the fines Halkbank faces, won’t be discussed until Brunson has been released.

The rejection could lead to the US imposing additional sanctions against Turkey sometime this week, the Journal reported. Treasury Secretary Steven Mnuchin said Thursday that the United States is willing to do so if Brunson isn't released.

"A real NATO ally wouldn't have arrested Brunson in the first place," the senior White House official told the Journal, referring to Turkey's membership of the North Atlantic Treaty Organization.

The administration wants Ankara to release Brunson and other citizens it holds on disputed terrorism charges, as well as three Turkish nationals who work for the U.S. government.

The Trump administration imposed sanctions against two top Turkish officials earlier this month, and Treasury Secretary Steven Mnuchin said the U.S. was prepared to take tougher steps.

On Friday, U.S. President Donald Trump called the charges against Brunson "phony," adding: "We are not going to take it sitting down."

Washington and Ankara have been in prolonged negotiations over a major fine against Halkbank, but the talks stalled. Halkbank is also facing further investigation in the U.S.

Halkbank recently complied with a subpoena request from the U.S. Treasury’s Office of Foreign Assets and Control, but its response was deemed insufficient, the White House official said. Washington has told Ankara that Halkbank must comply properly with the U.S. legal process before any discussions about relief could be entertained, the official told the Journal.